Isoquercitrin, also called isoquercetin, is a compound present in plants such as Houttuynia cordata, Apocynum venetum L (yunlong tea), cucumber, cotton, white clover, and mulberry. In addition to antioxidative (anti-fading, flavor change-inhibiting), UV-blocking, and metal chelating actions, this compound is considered to have pharmacological actions such as diuretic, anti-inflammatory, capillary-strengthening, vitamin P-like, and other like actions. Therefore, isoquercitrin is a useful compound that has been used, not only as an anti-fading agent for colored beverages and a flavor change inhibitor for food products, but also as an ingredient for cosmetics and health foods.
For isoquercitrin to exhibit anti-fading or flavor change-inhibiting action, it has to be used in a concentration of at least 0.001 w/v %. However, isoquercitrin dissolves in water at ambient temperature in a concentration of about 0.001 w/v % at most. Therefore, it has been impossible to practically apply isoquercitrin to aqueous products.
To solve this problem, as a method for producing α-glycosylisoquercitrin with high water solubility and the actions derived from isoquercitrin, a method comprising the step of transferring a glucose residue of a substrate to the site of a glucose residue of isoquercitrin using a glycosyltransferase has been previously proposed (Japanese Unexamined Patent Publication No. 213293/1989). α-Glycosylisoquercitrin thus obtained has been widely used as enzyme-treated isoquercitrin in the field of food products, fragrances, cosmetics, quasi-medical products.
α-Glycosylisoquercitrin is produced through a series of reactions such as a generation of isoquercitrin from rutin, and a glycosylation by glycosyltransferase treatment, etc. Enhancing the reaction yield or making the production more efficient is a task that people in the art still have to deal with and improve upon. As a method to accomplish the task, there are a method comprising the step of producing isoquercitrin in the presence of an organic solvent mixture (Japanese Unexamined Patent Publication No. 528133/2002) and a method comprising the step of enhancing reactions by albumin, silk protein, and the like (Biocatalysis and Biotransformation, Vol. 14 (1996): 113-123).